During a previous study, we observed that wild type human rotavirus, strain D, induced diarrheal illness in adult volunteers. Also in this study we identified serologic correlates of resistance. In a subsequent study, human rotavirus, Wa strain (type 1), adapted to growth in African green monkey kidney cell cltures was shown to infect susceptible volunteers. The failure of adult volunteers with a low level of pre-challenge serum antibody to develop diarrheal disease suggested that the Wa rotavirus mutant was attenuated compared to the wild type D rotavirus. This preparation of Wa strain cannot be used in further studies becasue a simian foamyvirus was subsequently identified in the seed used to prepare it. Also 3 volunteers developed a low-level rise in serum transaminase 10 days after administration of virus; the etiology of these rises has not yet been explained. Because of these problems, an additional suspension of the Wa human rotavirus is being prepared in pre-tested, adventitious virus-free African green monkey kidney cell cultures. It is planned that this virus suspension will be evaluated in susceptible adult volunteers. Efforts are also under way to adapt human rotaviruses to growth in human or rhesus monkey diploid cells; if successful, vaccines could be prepared in these virus-free cells.